The present invention relates generally to data transmission over wireless channels, and more specifically to wireless Internet access using cable-compatible modems.
The present invention is a method and apparatus for providing upstream data transmission for Internet access over wireless channels using standard or modified modems developed for Internet access over cable channels. At present, most households with Internet access use telephone modems and telephone lines to establish communication with an Internet service provider (xe2x80x9cISPxe2x80x9d) and access the Internet. The data rate over telephone lines is limited due to limited bandwidth. High speed wireless Internet is available using Local Multipoint Distribution Service (LMDS), but this approach requires a license from the Federal Communications Commission (FCC) and is relatively expensive. Furthermore, the service is not generally available. Therefore, providing Internet access over cable communications system has become an attractive alternative: greater bandwidth is available to provide high data rates, and many households are already connected to a local cable provider.
However, providing full-duplex Internet access via cable requires both a forward channel from the cable provider to the subscribing household (also known as the downstream direction), and a reverse channel from the subscribing household to the cable provider (also known as the upstream direction). Internet data such as web page content is transmitted downstream to the subscriber, and the subscriber transmits data such as requests for web page access or email upstream to the service provider. The subscribing household is equipped with a cable modem with a computer or in a set-top box, which receives and demodulates the downstream data, and modulates and transmits the upstream data.
Although Internet access over cable provides increased speed over telephone access, it is not ideal. Cable systems were originally developed to send information only in the downstream directionxe2x80x94to send television programming from the cable service provider to the subscribing household. Although some cable systems have been modified for upstream transmission, many have not, and must use an alternative such as telephone lines for upstream transmission. Furthermore, some households do not have access to cable. Finally, since local cable systems are by nature monopolistic, competition among ISPs providing service over cable is limited.
For this reason, wireless Internet access is an attractive alternative to access over cable. Recently, ISPs have attempted to provide wireless access at data rates comparable to the data rates available over cable. The current approach to xe2x80x9cwireless cablexe2x80x9d has been to use multichannel multipoint distribution service (xe2x80x9cMMDSxe2x80x9d) channels. The MMDS channels were originally conceived as an alternative means to cable for providing television service in remote areas. A limited number of wireless channels, with relatively small geographic coverage, were designated for MMDS and auctioned off by the FCC. MMDS proved commercially unsuccessful as a means for providing television service, but some ISPs have purchased the licenses and put the channels to use for providing Internet access over this xe2x80x9cwireless cable.xe2x80x9d The signals over wireless cable are processed to mimic cable signals, so that, unlike LMDS, standard cable modems may be used. The processing is performed using additional equipment, usually in an outdoor unit (xe2x80x9cODUxe2x80x9d) at the subscriber household.
However, MMDS is not entirely satisfactory. The MMDS wireless cable uses 6 MHZ television channels for upstream and downstream transmission. These channels are relatively closely spaced in frequency. Because the channels are so closely spaced in frequency, a diplexor is required at each household to separate the upstream transmission path and the downstream receive path. Thus, the processing equipment required, including the diplexor, is relatively expensive and cumbersome. Also, the up to two 6 MHZ channels which may be allocated to the reverse direction are the maximum frequency spectrum, which is a significant limitation. In addition, because the MMDS channels are licensed, the ISP must go to the expense of purchasing a license which could cost in the millions of dollars.
The use of unlicenced channels has generally been unattractive for wireless cable. Unlicenced FCC channels typically require the use of spread spectrum. However, spread spectrum is impracticable for the upstream channel, due to the incompatibility of the bursty nature of upstream communications and the synchronization time required for spread spectrum signals. With respect to the downstream channel, spread spectrum is impracticable as well. One approach to spread spectrum is frequency hopping, in which the center frequency of the signal changes in a pseudo-random fashion, at a rate which is less than the bit rate. Frequency hopping cannot be used for downstream transmission because continuous transmission is required in the downstream direction. Another approach to spread spectrum is direct sequence, in which the signal is phase-modulated very rapidly, relative to the bit rate, in a pseudo-random fashion. However, the complex modulation format used for downstream communications would make the use of direct sequence signals complicated and expensive. In addition, commonly employed methods for producing direct sequence signals would occupy a substantial portion of the available bandwidth without permitting the use of code division multiple access (CDMA), and would make the receiver vulnerable to narrowband interference.
There is therefore a need in the art for high-speed wireless Internet access which uses relatively simple, inexpensive equipment in addition to a cable modem or set-top box. Preferably, the Internet access can be provided over wireless channels without the purchase of a license and not requiring the use of spread spectrum.
In summary, the present invention is an apparatus and method for upstream data transmission in a system which provides Internet access using unlicenced wireless microwave channelsxe2x80x94that is, wireless microwave channels which may be used without a license from the FCC. The system uses widely-separated frequency channels for upstream and downstream transmission. Compliance with the FCC requirements is provided through an upstream modulation scheme that controls transmission power levels, while transmitting at the unspread (information) bandwidth, without the use of spread spectrum. Furthermore, the upstream and downstream modulation may be constrained to meet the requirements for a standard cable modem, including a cable modem meeting the DOCSIS standard, the contents of which is hereby incorporated by reference. The use of wireless cable rather than standard cable is transparent to the user.
The upstream portion of the system may be used in a two-way wireless system with both upstream and downstream channels on wireless cable. Alternatively, the upstream portion alone may be used to provide the upstream channel in a cable system which has not been upgraded to include an upstream channel within the cable, sometimes referred to as a xe2x80x9clegacy cable network.xe2x80x9d
An upstream signal, consisting of modulated data, is generated by the cable modem in response to a request from a user, for example to send email or to download a particular webpage. The upstream signal is located within the 5-42 MHZ frequency band assigned by most cable operators for upstream data transmission, referred to as the xe2x80x9cupstream frequency band.xe2x80x9d The upstream signal itself does not occupy the entire 37 MHZ bandwidth, but is contained within a 200 KHz-3.2 MHZ channel within the upstream frequency band.
The upconversion in the subscriber ODU is provided by an upstream frequency translator. The frequency translator upconverts the entire upstream band, 5-42 MHZ, output by the cable modem to a 37 MHZ sub-band within a wireless band. The wireless band is a range of frequencies, such as 2.4-2.4835 GHz, suitable for wireless transmission. The upconverted upstream signalxe2x80x94the xe2x80x9cwireless upstream signalxe2x80x9d xe2x80x94is then located in a portion of each upconverted 37 MHZ sub-band. For example, if the upstream signal originally occupied the frequencies 30-32 MHZ, and the band 5-42 MHZ is upconverted to 2.4 GHz-2.437 GHz, the wireless upstream signal will be located at 2.430 GHz-2.432 GHz. The wireless upstream signal is then provided to an antenna for wireless transmission.
The frequency translator may upconvert the upstream band to more than one 40 MHZ sub-band within the wireless band: for example, in the band 2.430-2.4835 GHz, the upstream band could be upconverted to both the sub-band 2.4-2.437 GHz and the sub-band 2.4435-2.4831 GHz. The subscriber ODU would then produce the wireless upstream signals at multiple sub-bands for transmission. The headend could then select for processing the sub-band with the least interference, or could combine sub-bands to increase the signal-to-noise ratio.
The wireless band is preferably in an unlicenced channel-a range of transmission frequencies for which the FCC does not require users to purchase a license. The average field strength of the transmitted signal is constrained to be less than a maximum value. If the field strength is properly constrained, it is not necessary to use spread spectrum modulation to satisfy FCC requirements for unlicenced channels. In the present invention, the wireless upstream signal is an unspread signalxe2x80x94i.e., a signal which has not been spread using means such as direct sequence or frequency hoppingxe2x80x94with its average field strength constrained through either controlling the duty cycle of the transmitted signal, using a calibrating circuit which is responsive to detected field strength, or both. Because the frequency translator does not spread the upstream signal, the bandwidth of the wireless upstream signal output by the frequency translator is approximately equal to the bandwidth of the upstream signal input to the frequency translator.